The goal of this research effort is to understand how an important type of white blood cell, called a T lymphocyte, recognizes the presence of a microorganism or cancer cell in the body, or inappropriately recognizes a normal component of the body (an "auto-antigen"). This research examines this question at the protein, and tissue levels, to provide a detailed understanding of how the substances (antigens) making up these microorganisms, cancer cells, or normal self-components, are made visible to the defending T cells or the auto-reactive T cells that cause disease. To accomplish these goals, we have examined the structure and function of special cellular proteins called major histocompatibility complex (MHC) molecules that are essential for antigens to be recognized by T cells. Specific questions involve understanding how structural differences in the MHC molecules of different individuals affect the ability of these proteins to stimulate T cells, the exact nature of the antigens bound to the MHC molecules, the events within a cell that bring the antigen and MHC molecule together to form the complex needed to activate a T cell, and the cellular distribution of the antigenic complexes within the body. These events are termed antigen processing and presentation. By understanding these events, we will be able to identify the most important components of microorganisms or cancer cells to use in protective or therapeutic vaccines, the best form in which to expose the body to these substances to stimulate an effective immune response, and possible ways in which to interfere with this recognition system, so that we can interrupt unwanted responses to "self" that cause autoimmune diseases such as multiple sclerosis, diabetes, or rheumatoid arthritis. During the past year we have generated and characterized novel reagents that have enabled us for the first time to directly visualize the expression of specific peptide:MHC molecule complexes on various cells in intact lymphoid organs, and to quantitate such complexes on these cells in vitro. We have utilized related methods to reveal the key role of dendritic cells in infection-stimulated, T-cell independent production of the cytokine IL-12 that contributes to effective immunity to intracellular pathogens.